Mainly, technical-grade vinyl chloride (VCM) is produced by the non-catalytic and incomplete thermal decomposition (pyrolysis) of gaseous 1,2-dichloroethane (EDC), in the course of which hydrogen chloride (HCI) is formed as a coupled product.
EP-0276775-B1 discloses that standard commercial, non-catalytic processes are performed at a pressure of below 1 to 3 Mpa and at a pyrolysis zone outlet temperature of 450.degree. C. to 600.degree. C. The inlet temperatures in the pyrolysis zone range from 200.degree. C. to 300.degree. C.
In U.S. Pat. No. 5,488,190 a process is described which prevents the formation of by-products during the pyrolysis of EDC, the heat being transferred via a tube wall in direct contact with a gaseous or solid matter which has been admixed with the EDC and heated to a high temperature. During this process, the EDC is only evaporated in the subcritical pressure and temperature range.
DE-A 35 43 222 describes an improved process for producing vinyl chloride by thermally cracking 1,2-dichloroethane to obtain gaseous EDC from liquid EDC in the subcritical range, as can be seen in FIG. 1 of the patent.
In the commercial process, the pyrolysis zone is designed as a burner which is heated with natural gas or fuel oil. The burner's combustion chamber has one or more tube coils in which EDC pyrolysis occurs.
The residence time in the pyrolysis zone and the temperature are selected to ensure that on the one hand, as few undesirable by-products as possible are formed, such as soot (coke), chlorinated and unsaturated hydrogen chlorides and benzole, and on the other hand, to ensure for economic reasons, that the EDC yield is at least 50%
The cause of soot formation can by no means be solely ascribed to the relatively high temperatures in the pyrolysis zone in standard commercial processes. The EDC, which is usually in liquid phase, is evaporated and superheated if necessary, prior to pyrolysis.
Evaporators and, for example, boiling reactors have a decidedly unfavorable residence time pattern as regards substances which have a tendency to decompose as in agitated boilers. As a result, precursors, which encourage soot formation, are formed during EDC evaporation before the EDC reaches the actual pyrolysis zone.
This is substantiated, for example, by the fact that in the known process, which involves cracked gas heating and external evaporation (heat recovery) and which is applied in the EDC/VDM production plants constructed by the patent applicant, a blow-down flow must be continually drawn off to prevent premature contamination of the evaporator and the pyrolysis tubes. The blow-down flow contains decomposition products which are rich in carbon and which have coalesced to form larger particles.